In optical communication, as large-capacity information communication means, a WDM (Wavelength Division Multiplex) scheme for causing wavelength multiplexed light obtained by multiplexing a plurality of lights having different wavelengths to pass through a transmission line formed by an optical fiber and an optical component and transmitting the wavelength multiplexed light is widely used. An optical transmission system adopting the WDM scheme usually includes an optical amplifier that amplifies the wavelength multiplexed light.
In particularly, a submarine optical cable system used in international communication and the like includes, in each relay section of approximately 50 to 100 kilometers, a submarine relay device including an optical amplifier. At initial introduction time of the optical transmission system, in some case, a part of wavelengths in a wavelength band amplified by the optical amplifier, that is, an amplification band of the optical amplifier are used for transmission of signal light and the other wavelengths are not used. In this case, in the optical amplifier designed under an assumption that wavelength multiplexed light obtained by multiplexing signals in all wavelength bands of the amplification band is amplified, optical power per wavelength band of the wavelength multiplexed light is excessive. Such excessive optical power causes a nonlinear optical effect in a transmission line and deteriorates transmission quality of the wavelength multiplexed light.
Therefore, dummy light not having a signal component is sometimes included in a wavelength band not used for transmission of signal light in the wavelength multiplexed light propagated in the transmission line. It is possible to obtain a desired transmission characteristic from the initial introduction time of the optical transmission system by transmitting, using the dummy light, the wavelength multiplexed light obtained by multiplexing signal lights in all wavelength bands of the amplification band of the optical amplifier. A method of using CW (Continuous Wave) light as the dummy light or a method of using ASE (Amplified Spontaneous Emission) light as the dummy light has beenproposed. See, for example, Patent Literature 1.